A programmable read-only memory (PROM) is a non-volatile data storage, wherein programming of data may be performed after the device has been manufactured. Typically, PROMs are manufactured blank and depending on the technology, the PROM-device is programmed at wafer level, at a final testing stage or in system. Classical PROM-devices log each bit in a memory cell comprising a fuse or an anti-fuse. Typically, the default setting of each memory cell is a logical value identical to “0”. By blowing the fuse or anti-fuse irreversibly, this value may be set to “1”. Blowing a fuse opens a connection while programming an anti-fuse closes the connection. Because it is impossible to “un-blow” the fuse, data is stored permanently.
A different approach to a PROM-device is a carbon nanotube based memory cell, as exemplarily disclosed by Furukawa in U.S. Pat. No. 7,668,004. This document teaches a memory cell 2 as shown in the simplified circuit diagram of FIG. 1. The memory cell 2 comprises a MOS transistor 4 that is coupled to a word line WL and to a bit line BL. Further, a sense amplifier 6 is coupled to the bit line BL and to a reference voltage VREF. The channel of the MOS transistor 4 is coupled to a carbon nanotube element 8 comprising a storage electrode that is coupled to a storage node 10 and to a release electrode that is coupled to a release node 14 that is further coupled to a release line RL. A carbon nanotube 16 is coupled to a reference node 12.
Within the context of this specification a carbon nanotube 16 will be referred to as a nanotube only. Preferably, single walled carbon nanotubes having the typical cylindrical nanostructure which is made from a honeycomb crystal lattice of graphene which is a hexagonal carbon structure are applied.
The nanotube 16 has two possible states which are illustrated in FIG. 2a and FIG. 2b. In principle, the nanotube element 8 is a switch, wherein by way of an example only, the status in FIG. 2a corresponds to an ON-state representing a logical “1” and the status in FIG. 2b represents the OFF-state and corresponds to a logical “0”. There is a small capacitance between the nanotube 16 and the storage terminal 18 and the release terminal 20, respectively. By applying a suitable voltage between either the storage node 10 and the reference node 12 or the release node 14 and the reference node 12, a respective switching state may be realized. Due to attractive Van-der-Waals forces between the nanotube 16 and the respective terminal 18, 20, the nanotube 16 is bound to either one of the electrodes 18, 20 and is held in its respective switching state.
In a memory cell 2 of a PROM-device, the connection or disconnection between the word line WL and the bit line BL is set permanently. FIG. 3 is a simplified detailed view to a couple of memory cells 21 . . . 26 of a PROM-device 6. The memory cells 21 . . . 26 are coupled between the respective bit lines BL1 . . . BL3 and word lines WL1 and WL2. Each bit line BL1 . . . BL3 comprises a protective resistor 30. Further, each memory cell 21 . . . 26 comprises a switch 32, e.g. a fuse or anti-fuse element or a nanotube element 8 as exemplarily shown in FIGS. 1 and 2. For reading out a memory cell 21 . . . 26, a high level is applied to the selected word line, e.g. WL1. Now, access is provided to all memory cells 21 . . . 23 that are coupled to this word line WL1. After this addressing step, a conductive memory cell, e.g. memory cell 22 provides a high level to the assigned bit line, i.e. BL2. In order to protect the other memory cells, i.e. memory cells 21 and 23 from the high level of word line WL1, every memory cell 21 . . . 26 comprises a diode 34.
However, the data retention times of PROM-devices using a carbon nanotube based switch 32 is not sufficient, especially for data archives. Further, the PROM-device comprises a high number of different elements, e.g. switches 32, diodes 34 and resistors 30. This leads to a complex production process.
It is an object of the present invention to provide an improved PROM-device and an improved method of operating a PROM-device.